In the production of paper from wood fibers, the wood fibers must be freed from the raw wood. In one widely used method, this is accomplished by cooking the wood fibers in a solution until the material holding the fibers together, lignin, is dissolved. In order to achieve rapid and uniform digestion by the cooking liquor, the wood, after it has been debarked, is passed through a chipper which reduces the raw wood to chips. The raw chips from the chipper may contain many overthick and oversized and grossly overthick and oversized wood chips. The raw chips also contain fines, dust-like material having little useful fiber; and pins, material slightly larger than fines, having some useful fiber which can be tolerated in the pulping process at an acceptable proportion.
Existing processes for preparing pulpwood for digestion after it has been reduced to chips by a chipper have normally involved multiple screening steps to separate out the acceptable chips from the oversized and grossly oversized chips. The overthick and oversized and grossly overthick and oversized chips are normally then reprocessed through a rechipper, slicer or chip destructuring device to convert them into acceptable chips.
A recently developed device for processing wood chips includes closely operating rolls which are provided for supplying compressive forces to chips passing therebetween, wherein at least one roll has an aggressively contoured surface for causing chips to crack in the thickness dimension of the chip as compressive force is applied to the chip.
As disclosed in U.S. Pat. No. 4,953,795 to Bielagus, such a chip destructuring device may be used with oversized chips separated at a previous screening step, or the entire chip flow in a pulping operation can be processed through such an apparatus. Bielagus further discloses that in other applications it may be desirable to separate only the undersized chips from the total chip stream and then process both oversized and acceptable size chips through the apparatus. Bielagus points out that a high volume of chips can be passed through the chip destructuring apparatus, making it possible to process the entire chip flow in the pulp mill, potentially eliminating the need for screening out oversized chips. Bielagus teaches that if acceptable and oversized chips can all be passed through the apparatus, it is unnecessary to separate the over-large chips for separate treatment.
One known method of preparing wood chips for digestion in the pulping process employs a double gyratory or rotary wave-type screen having a first set of holes approximately seven-eights to one-and-a-quarter inch in diameter, and a second screen sized to remove fines with openings of approximately an eighth of an inch. The first screen removes a large portion of the over-thick and oversize chips and also a large portion of the acceptable chips. The reject flow from the first screen is all run through the chip destructuring device, the output of which is combined with that which passed the first screen to form a combined stream which is suitable for being converted to pulp at the same time.
Methods are needed for employing chip destructuring devices which will prepare wood chips for pulping with reduced cost, space and maintenance requirements.